1. Field of the Invention
The present invention relates to an apparatus for generating a signal indicative of the intensity of knock occurring in an internal combustion engine.
2. Description of the Prior Art
Strong knocks occurring in an internal combustion engine cause harm to the engine. However, it is generally known in the art that the engine output performance and the fuel consumption characteristics are optimized when the engine is running under a slight knock condition.
In view of the fact that the occurrence of knocking is greatly dependent on the ignition timing, various systems have been proposed wherein the ignition timing is retarded upon detection of knocking.
In order to accurately determine the ignition timing in accordance with the intensity of knocking, apparatus is provided for precisely determining the intensity of knocking occurring in the internal combustion engine.
One technique that has been proposed for generating an accurate knock signal is amplification discrimination, in which an output signal of a vibration sensor mounted on the engine is compared with a noise reference signal at a comparator.
The noise reference signal masks most of the sensor output signal so that only peaks in the sensor output signal which are assumed to be mostly due to knock-induced vibration rising above the noise reference signal contribute to the knock signal.
This type of knock signal generating apparatus results in an improvement in signal to noise ratio of the knock signal.
However, a difficult problem in such amplification discrimination is to eliminate the influence of the peaks of the sensor output signal upon the voltage level of the background noise signal.
In other words, there is a tendency for the peaks of the sensor output signal resulting from knocking falsely increase the voltage level of the background to noise signal, and undesirably reduce the apparent amplitude and the frequency of knock signal which is produced at the output of the comparator.